Aspects of the present disclosure describe systems, methods, and structures—including LiDAR—that employ multiple detectors that may determine multiple incident angles of multiple received radiation beams and advantageously do not require or employ phase shifters in illustrative embodiments and may instead—employ optical Fourier transform structures.
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8. An optical structure comprising:
a plurality of input waveguides;
a plurality of output waveguides;
a slab waveguide interposed between the input waveguides and the output waveguides; and
a waveguide-based lens formed in the plurality of input waveguides;
wherein said optical structure is configured to impart a focusing phase front on optical energy traversing the structure using the waveguide-based lens.
1. An optical device comprising:
an emitter structure configured to emit optical radiation into an interrogation region;
a plurality of collector structures configured to collect the emitted optical radiation, backscattered from objects located in the interrogation region;
a plurality of coherent receiver structures, a respective one for each individual collector structure, each respective coherent receiver structure configured to receive the backscattered optical radiation collected by its respective collector structure and produce signals indicative of the phase and amplitude of the received optical radiation; and
a processing structure configured to digitally determine offset angles to the objects using the phases and amplitudes of the received optical radiation.
2. The optical device according to
3. The optical device according to
4. The optical device according to
5. The optical device according to
6. The optical device according to
7. The optical device according to
9. The optical structure of
10. The optical structure of
11. The optical structure of
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This application claims the benefit of Untied States Provisional Patent Application Ser. No. 62/520,271 filed 15 Jun. 2017 which is incorporated by reference as if set forth at length herein.
This disclosure relates generally to integrated optical systems, methods, and structures useful—for example—in applications involving imaging, ranging, sensing, and optical communications.
As is known, the ability to precisely direct radiation beams at a desired area/angle and detect backscattered radiation is of considerable importance to many contemporary technologies including—but not limited to—imaging, ranging, sensing, measuring, detecting, and communications, which—in turn—find widespread applicability in scientific and medical instruments and systems, vehicular safety and control, robotic navigation, and optical communications.
One particularly noteworthy system employing the precise direction/detection of radiation beams is a LiDAR system. As presently provided in the art however, such LiDAR systems with conventional optical phased arrays typically employ optical phase shifters which are known to require an undesirably high-power budget, are optically lossy, and oftentimes require a large I/O count in implementation.
An advance in the art is made according to aspects of the present disclosure directed to systems, methods, and structures—including integrated photonics LiDAR—that may need to simultaneously determine incident angles of multiple received radiation beams.
In sharp contrast to the prior art—systems, methods, and structures according to aspects of the present disclosure advantageously do not require or employ active phase shifters in illustrative embodiments.
Viewed from one aspect, systems, methods, and structures according to aspects of the present disclosure may include an illustrative integrated optical device comprising: a emitter structure configured to emit optical radiation into an interrogation region; a plurality of collector structures configured to collect the emitted optical radiation backscattered from objects located in the interrogation region; a plurality of coherent receiver structures, a respective one for each individual collector structure, each respective coherent receiver structure configured to receive the backscattered optical radiation collected by its respective collector structure and produce signals indicative of the phase and amplitude of the received optical radiation; and a processing structure configured to digitally determine an offset angle to the objects using the phases and amplitudes of the received optical radiation.
Viewed from another aspect, systems, methods and structures according to aspects of the present disclosure may include an illustrative optical device comprising: an emitter structure configured to emit optical radiation into an interrogation region; a plurality of collector structures configured to collect the emitted optical radiation, backscattered from objects located in the interrogation region; an optical Fourier transform structure that projects a Fourier transform of the collected optical radiation; and a plurality of receiver structures, each receiver structure configured to receive a portion of the transformed optical radiation.
Finally, and viewed from yet another aspect, systems, methods and structures according to aspects of the present disclosure may include an illustrative optical device that advantageously provides a transform of optical radiation and may illustratively include: a plurality of input waveguides; a plurality of output waveguides; and a slab waveguide interposed between the input waveguides and the output waveguides wherein the optical structure is configured to impart a focusing phase front on optical field traversing the structure.
A more complete understanding of the present disclosure may be realized by reference to the accompanying drawing in which:
The illustrative embodiments are described more fully by the Figures and detailed description. Embodiments according to this disclosure may, however, be embodied in various forms and are not limited to specific or illustrative embodiments described in the drawing and detailed description.
The following merely illustrates the principles of the disclosure. It will thus be appreciated that those skilled in the art will be able to devise various arrangements which, although not explicitly described or shown herein, embody the principles of the disclosure and are included within its spirit and scope.
Furthermore, all examples and conditional language recited herein are intended to be only for pedagogical purposes to aid the reader in understanding the principles of the disclosure and the concepts contributed by the inventor(s) to furthering the art and are to be construed as being without limitation to such specifically recited examples and conditions.
Moreover, all statements herein reciting principles, aspects, and embodiments of the disclosure, as well as specific examples thereof, are intended to encompass both structural and functional equivalents thereof. Additionally, it is intended that such equivalents include both currently known equivalents as well as equivalents developed in the future, i.e., any elements developed that perform the same function, regardless of structure.
Thus, for example, it will be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the disclosure.
Unless otherwise explicitly specified herein, the FIGs comprising the drawing are not drawn to scale.
By way of some additional background, we note that integrated optical structures have attracted considerable attention for many applications including imaging, ranging, sensing, free-space communications, etc. This interest has further accelerated given recent utilization of advanced lithography and etching techniques originally developed for CMOS applications to the fabrication of such integrated optical structures.
Known further, phase shifters employed in contemporary optical systems require a relatively high, disadvantageous power budget or can have high optical loss. Eliminating the need for such phase shifters would represent a welcome addition to the art.
Accordingly, we disclose systems, methods, and structures that do not require active optical phase shifters to determine angles of received light (radiation). As will be readily appreciated by those skilled in the art, such systems, methods, and structures according to the present disclosure require much lower power and loss than those prior art systems employing active phase shifters.
As we shall show and describe, we disclose illustrative LiDAR systems to demonstrate our inventive concepts. We note that while we are using LiDAR illustratively, our disclosure is not so limited or exclusive to LiDAR applications and may find applicability in imaging, ranging, sensing, and optical communications. Note further that systems, methods, and structures according to aspects of the present disclosure may include integrated optical structures that may advantageously be CMOS-fabrication compatible, silicon nano-photonics.
In presenting our disclosure, we begin by noting that an illustrative coherent LiDAR system according to aspects of the present disclosure may include one or more antennae for emitting optical radiation and multiple antennae for collecting backscattered optical radiation. According to aspects of the present disclosure, in an illustrative coherent LiDAR system having a single emitter and multiple collectors, light is emitted from a single emitting antenna or, alternatively, from an optical phased array, into an interrogation area (environment) where it may strike objects and be backscattered from those objects and subsequently collected by multiple receiving collectors. The collecting antennae are in turn optically connected to a respective coherent receiver which produces signals indicative of the phase and amplitude of the backscattered light collected by the respective collector. These signals are subsequently processed such that an angle, and potentially distance, to the object from which light is backscattered. Of particular advantage, multiple objects may be simultaneously detected and their distance(s) and angle(s) relative to the emitter may be determined.
Illustratively, the emitter structure depicted in
The far field pattern (Fraunhofer range) of the emitted optical radiation will be understood by those skilled in the art to be the Fourier transform of the near field emission. For a relatively long and narrow emitter structure like that illustrated in
Turning now to
We note that in one illustrative embodiment, the multiple collecting structures may include waveguide grating antennae to collect backscattered light. The light so collected is directed to a photonic circuit—that may be as simple as a straight optical waveguide—and subsequently into multiple coherent receiver structures.
As illustratively shown in the figure, each of the collecting structures is associated with an individual coherent receiver structure that may include one or more photodetectors suitable for the detection of the backscattered radiation. Each respective coherent receiver structure is configured to receive the backscattered optical radiation collected by its respective collector structure, “beaten” with a portion of a local oscillator output signal (LO) and produce signals indicative of the phase and amplitude of the received optical radiation.
At this point we note that in a preferred, illustrative embodiment, the single emitter structure is substantially identical to each individual one of the plurality of receiver structures although it need not be. Of particular interest to those skilled in the art, such emitter may include an optical phased array.
Note further that as shown in this figure, since multiple objects are shown as illuminated by the emitted optical energy within the transmitted far field line, one needs to distinguish each object by their angle relative to the collector structure as well as their distance(s) from the collectors.
As we shall show and describe, systems, methods, and structures according to aspects of the present disclosure advantageously provides for detecting and distinguishing objects and their relative direction(s) and distance(s) by any one of a number of techniques including digital beam steering, optical Fourier transform, and lens(es) or other optical elements preceding collector structures in the optical path of the backscattered light. Regardless of the method of converting light from the pupil plane to the image plane (to measure object properties as a function of angle), there are several techniques that can be used to measure properties of the object(s). Object properties of interest may include the range to the object(s), the velocity of the object(s), the vibration profile of the object(s), or the complete amplitude and phase of the light field projected by the object(s).
One method, known as frequency modulated continuous wave (FMCW) detection with source modulation involves directly modulating a laser source frequency with either cavity length change, filter frequency adjustment, current injection or any other method that slightly changes the central frequency of the laser as a function of time. The laser frequency is ramped up/down and the amount of the delay in the backscattered signal return and the Doppler shift due to the movement of the target determine the frequency of the detected RF signal. Therefore, by measuring the frequency in each time section both the distance and the speed of the object are determined
Alternatively, instead of relying on the frequency or phase of the light sent out, the source amplitude can be modulated with a chirp function. Note that cross-correlation of the returned backscattered signals and the local oscillator are used to determine the total travel time of the signal to the object and back. Moreover, to eliminate the effect of small movements of the object and any slight changes in the optical path due to turbulence, I/Q detectors can be used to eliminate the effect of the optical phase of the return signal and extract only the amplitude of the returned light.
Digital Beam Steering
One method of determining the angle of any received light utilizes a respective coherent receiver structure for each collector structure and sampling the phase of any electrical beat frequencies that provide information about an interrogated/illuminated object (target).
With reference to
We note that this phase detection can be achieved in any pseudo-heterodyne configuration that produces an electrical beat frequency such as frequency-modulated continuous-wave (FMCW) LIDAR. Consider this case where due to the time delay between the received signal and the LO, there is an electrical beat frequency at the detector that is related to the time-of-flight distance between the system and the target. Due to multiple targets being illuminated, multiple beat frequencies arise at each photodetector as shown below:
where t is time, PRX and PLO are the optical powers of the received signal and the local oscillator, ωRX and ωLO are instantaneous received signal and local oscillator optical frequencies, and θRX and θLO are the optical phase of the received signals and local oscillator, respectively.
To determine the angle to the object, the phase difference of the frequency of interest between collector antenna elements can examined. This is because the received signal can be viewed as a collection of plane waves of different angles and frequencies, each plane wave angle for a different object in an interrogation region.
The individual incoming plane waves have slightly different frequencies due to the frequency modulation of the light source and the fact that the total travel time from the emitting aperture to the object and back into the collecting apertures depend on the exact distance each object is located at away from the device.
When comparing two adjacent photodetectors, it is clear that each of them receives all plane waves from all around it and creates a continuum of RF frequencies when the collected light is beaten against the local oscillator. Thus, the difference between adjacent detectors is that at each frequency of the RF spectrum, which normally corresponds to backscattering from one spot in the distance, there is a constant jump in phase going from each detector to its neighboring elements. Therefore, looking at all the photodetectors (or some subset of them), and observing the amount of the phase difference between neighboring elements, one may determine the angle at which a plane wave arrived is given by the formula:
where θi is the angle to target i, λ is the wavelength of light in the medium of propagation, Δϕi, is the phase difference between receiver elements for frequency (ωRXi−ωLO) for object i, and d is the spacing between the collector elements. In this equation, can have multiple solutions (for multiple values of n) as long as the term inside the brackets stays smaller than unity. Therefore, for arrays with large separation between the receiving elements (d>λ/2), there is an ambiguity in the angle of the incoming wave as it is down-sampled by a spatially sparse array.
Much like an array factor, this formula only has a single solution when the receiver elements (collector structures) are spaced apart by a distance less than λ/2. One can also think of this as being the Nyquist criteria but in the spatial domain. This can be circumvented in several ways such as aperiodic spacing between structures such as that illustratively shown in
Optical Fourier Transform
Another method to split the multiple received signals into different “angle bins” is to use an on-chip optical Fourier transform and detect each spatial frequency with a detector in the optical domain. As known in Fourier optics, a lens creates a Fourier transform of the wave distribution on its other side.
For example, as shown in
Especially, when the object plane (left side) is at a focal distance (z=f), the above equation simplifies to a Fourier transform:
As can be seen from
In our devices, a plane wave that is incident to the collector elements creates a linear phase distribution in the collector elements. As may be observed in
Stated alternatively, the g(x) function for a collection of guided mode inputs becomes a discrete function g(i). The angle of the incoming wave is now translated to the angle of a complex function g(i). If the waveguide collectors are at a distance d from each other, an incoming wave with an angle θ with respect to the normal leads to a phase function
The Fourier transform property of the lens leads to G(x″) having a peak at x″=fd cos(θ).
To realize an on-chip, all planar configuration for such a system, we first apply a focusing phase front to the optical energy (light) and then propagate it for a length at least equal to the focusing distance of that phase front. More specifically, an optical Fourier transform can be achieved by creating an on-chip lens by propagating the light from the grating waveguides in a slab waveguide medium and implementing the phase function of a lens unto the in-plane propagating light with perturbations to the local effective index of the slab with a photonic crystal or an added material effecting the evanescent tail of the slab guided mode.
At this point those skilled in the art will readily appreciate that light traversing the incoming waveguides (incoming waveguide mode) are directed into the slab waveguide (slab mode propagation) where it undergoes the effect of the planar lens. Consequently, and according to aspects of the present disclosure, such affected light will exhibit a converging and/or focusing phase front (i.e., hyperbolic-shaped) and be directed in the slab to the output waveguides where it subsequently exits the overall structure. As will be appreciated by those skilled in the art, a length of the slab waveguide through which the focusing wavefront propagates will be at least as long as focus point of the wavefront.
To find the optimal shape of the planar lens, the effective index of the planar slab mode in the regions that a second film (planar lens) is present (n2) and the other regions (n1) should be considered.
Turning now to
which shows that the higher the index contrast between the two slab regions is, the flatter the lens surface will be.
Note that the index contrast between the two regions depends on the confinement of the light in the slab region and the type of perturbation used for index modification. For example, if the light is propagating in a high index slab mode (for example 220 nm silicon device layer) and a layer of silicon nitride is added as a perturbing top layer (
To make the lens easier to fabricate (at the expense of wavelength range, the function d(y) can be flattened by subtracting multiples λ/Δn from the lens width. Such a configuration is shown illustratively in
Another method of performing an optical Fourier transform on chip instead of the above-described planar lens perturbation technique involves direct phase perturbation on the waveguides. In this method, a converging and/or focusing phase function necessary for focusing the wave front is superimposed on the optical waveguides carrying the collected light after the collectors.
More particularly, a set of tunable phase shifters (electrooptic or thermal) or static delay lines can be created such that a central waveguide experiences the greatest amount of phase shift and the delay is reduced following an analytic function for all other waveguides.
In an ideal hyperbolic case, if the separation of the waveguides is d and the effective index of the slab
propagation mode is n it follows that the phase function imposed on waveguide i should be:
ϕ(i)=ϕ0−n(√{square root over ((di)2+f2)}−f)
Similar to the evanescent perturbation case illustrated in
It is noteworthy that due to the Petzval field curvature—similar to images shaping perfectly in the human eye—the focal array of photodetectors should be placed in a circular pattern rather than a plane. The radius of the Petzval circle is approximated as
As shown in
Alternatively, instead of creating a planar, on-chip lens and detector array, one can use a conventional glass lens for the Fourier transform operation. As can be seen in
The signal, on the other hand, is collected at the individual detectors based on the direction of the incoming LiDAR signal which determines the phase distribution of the signal on the collection gratings. The signal from each LiDAR direction will interfere with its portion of the local oscillator on the corresponding focal plane photodetector and creates a beat signal corresponding to the round-trip time the light has traveled from the source and back to the detector array
Another similar alternative to this scheme is a case that instead of emitters emitting the light upwards as shown illustratively in
Focal Plane Array Approach
Instead of using a lens at the output of the collecting gratings, the lens can be utilized before the gratings. In this approach, the gratings are placed at the focal plane of a lens—which could be cylindrical, anamorphic, Fresnel, diffractive, or conventional. The incoming planes waves (scattered from an object at a certain angle and distance from the chip) will be focused by the cylindrical lens. The grating collectors located at the focal plane of the lens each receive only a small range of angles in the lateral direction. The cylindrical lens, being uniform in the longitudinal direction, does not affect the longitudinal direction of the incident waves, which is controlled by the wavelength both on the send and receive gratings. At the end of these collecting gratings, each being responsible for a certain transverse angle, a collection of receivers is placed with each one receiving a copy of the local oscillator. The phases of the local oscillator delivered to each receiver unit can be adjusted with a phase shifter, but this is not necessary. If the branches of the local oscillator delivered to all receivers are not synced, there could be an offset in the phase of the RF signals generated at each receiver. As the receivers in this case are independent from each other and each is responsible for a certain angle, the phase of the generated RF signal does not have a significance in many heterodyne or homodyne.
As can be seen from the inset of
As can be seen from the
At this point those skilled in the art will readily appreciate that light traversing the incoming waveguides (incoming waveguide mode) are directed into the slab waveguide (slab mode propagation) where it undergoes the effect of the planar lens. Consequently, and according to aspects of the present disclosure, such affected light will exhibit a hyperbolic-shaped phase front (or any desired focusing phase front) and be directed in the slab to the output waveguides where it subsequently exits the overall structure. As will be appreciated by those skilled in the art, a length of the planar or waveguide lens, through which any focusing wavefront propagates will be at least as long as the focus point length of the focusing wavefront.
As will now be readily appreciated by those skilled in the art, systems and structures—and methods employing same—according to aspects of the present disclosure—may advantageously be integrated on a chip such as a silicon photonics based chip with embedded or external photodetectors (that may be Silicon, Germanium, InGaAs, HgCdTe, or other).
Of further advantage, CMOS-compatible materials and methods as are known in the art are perfectly suited to fabricate structures and circuits consistent with the present disclosure. Such circuits may advantageously include silicon and/or silicon nitride and/or silicon dioxide. Other alternative materials, methods, and structures consistent with aspects of the present disclosure include glass-based or III/V based platforms as well.
Finally, we note that receiver structures employed may include an array of single-ended detectors, balanced detectors (2 photodiodes), or IQ detectors (2 balanced detectors with 4 detectors total). As will be understood and appreciated by those skilled in the art, balanced/IQ detectors may use 180° hybrids or an array of quadrature receivers using 90° hybrids or a combination thereof. Lastly, we note that an illustrative structure such as that illustrated in
At this point, while we have presented this disclosure using some specific examples, those skilled in the art will recognize that our teachings are not so limited. Accordingly, this disclosure should be only limited by the scope of the claims attached hereto.
Poulton, Christopher, Watts, Michael, Russo, Peter, Vermeulen, Diedrik, Hosseini, Ehsan, Byrd, Matthew
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